It is proposed to study the actomyosin ATPase reaction with a combination of oxygen labeling and fast kinetic methods. Multiple oxygen labeling methods have proven to be a unique asset in brinding oxygen labeling data into line with rapid kinetic data (Greeves et al. 1979). In addition, oxygen labeling has shown that most of the ATP hydrolysis takes place within the actomyosin complex, not through repetitive cycles actin-myosin dissociation and reassociation, even at physiological ionic strengths (Midelfort, et al., 1980, in press). These data, obtained under steady state conditions, leave several questions unanswered: 1) while we have measured the ratio between reversible hydrolysis and product dissociation in the myosin-products complex, the absolute rate constants can only be measured under single turnover conditions (times equals 0.03 - 2 seconds). Thus, we propose to compare directly the rapid burst kinetics with the oxygen exchange kinetics. 2) The role of a number of regulatory phenomena on the interactions between actin- and myosin will be assessed. For example, the role of Ca ions ion, phosphorylation by myosin light chain kinase, removal of the DTNB light chain, and decoration of actin filament with tropomyosin on the catalysis of the actomyosin complex will be assessed. In summary, multiple oxygen labeling is a valuable and perhaps unique tool in quantitating ATP hydrolysis within the actomyosin complex.